1. Field of the Invention
The present invention relates to a wireless communication system, and in particular to a probing method for fast handoff between access points.
2. Description of the Related Art
The de facto standard for wireless local area networks (WLAN) is Institute of Electrical and Electronics Engineers (IEEE) standard 802.11 communication system. This standard enables low-cost and effective WLAN services. An unlicensed spectrum (2.4 GHz in 802.11b/g and 5 GHz in 802.11a) used by the IEEE 802.11 standard allows the deployment of low-cost WLANs. A Medium Access Control (MAC) protocol provides high-bandwidth communication systems with bandwidths of up to 11 Mbps in 802.11b and 55 Mbps in 802.11a.
To date, WLAN devices have been integrated into mobile PCs and other mobile devices. Moreover, the 802.11 standard is being incorporated into a fourth generation (4G) cellular system because of the high bandwidth of WLAN. In fact, WLANs using the 802.11 standard are successfully deployed in public areas such as airports, hotels, universities and shopping centers.
However, as user mobility increases, the small cell size as defined by the 802.11 standard in WLANs can induce frequent handoffs, causing inevitable communication delays, because the handoff process includes finding a new best available access point (AP) and establishing association to that AP (layer-2 handoff). For Internet Protocol (IP) connectivity, an additional process of layer-3 handoff should be completed. Voice over IP (VoIP), one of the most popular applications of WLAN, requires a maximum end-to-end delay of 50 ms. Unfortunately, a majority of WLANs cannot complete the layer-2 handoff process in 100 ms. Moreover, handoff latencies between 60 ms and 400 ms are typical depending on the type and manufacturer of a wireless card or access point. Furthermore, a Probe Phase (the discovery of next AP) is known to be a dominating factor in handoff latency, accounting for more than 90% of overall handoff latency.
A proving latency is affected significantly by two parameters: probe count and probe-wait time. As defined by the 802.11 standard, the probe count equals the number of channels to probe, and the probe-wait time is between MinChannelTime and MaxChannelTime. MinChannelTime and MaxChannelTime are defined as being approximately 7 ms and 11 ms, respectively. Since the IEEE 802.11 standard does not specify which channels to probe, wireless cards use their own heuristic algorithms. These algorithms are categorized into full-scanning and observed-scanning. The full-scanning algorithm is a brute force algorithm that probes all legitimate channels (e.g., 11 channels in the United States). Observed-scanning restricts probing channels to a subset of legitimate channels which has been observed by previous probes. Independent channels are channels that provide enough frequency separations to co-locate several radio links without interference. Three channels (1, 6, and 11) are known to be independent in 802.11b. That is, in a proper deployment of 802.11b, the observed-scanning probes only these three channels, reducing probe count from 11 to 3. However, the observed-scanning algorithm can suffer when it must probe a long list of channels which happens when a sufficient number of independent channels are provided. For example, an emerging new WLAN standard known as the 802.11a standard, provides 12 independent channels for implementation in the United States. To benefit from an abundant number of channels, WLANs can use all 12 channels throughout the network. Therefore, the 802.11a standard will increase the number of used channels by 4 foldover the 802.11b standard, thereby increasing the probing latency of observed-scanning by a similar proportion.
Accordingly, a method for minimizing the probing latencies caused by the increasing channel numbers so as to provide a seamless handoff in WLAN systems is desirable.